This invention relates to fluorescent photosensitive vitroceramics and processes for making them. More specifically, this invention relates to fluorosilicate vitroceramics exhibiting both fluorescent and photosensitive properties. These fluorescent and photosensitive properties are imparted to the inventive vitroceramics by the inclusion of certain rare earths and certain photosensitizing metals in the vitroceramic composition.
A vitroceramic is a glass matrix having fine crystals precipitated therein. Vitroceramic material is obtained by first melting a glass, such as a fluorosilicate glass, in any conventional manner. The resultant glass is then subjected to a heat treatment at a temperature above the glass transition temperature, thereby preferentially precipitating small crystals. Once the crystals are precipitated, the material has been transformed from glass to a vitroceramic.
Generally, when crystals are precipitated in a glass, the optical transmission is significantly reduced because the crystals cause light scattering. However, if the precipitated crystals are very small (e.g., smaller than the wavelength of incident light), and, if the difference in refractive index between the crystals and the glass matrix is also small, the loss of optical transmission due to light scattering is substantially minimized.
Crystal precipitation can be controlled with nucleation seeds which serve as catalysts for the crystal precipitation process. The efficiency of a given catalyst depends on a number of factors, including the similarity between the catalyst's own crystal structure and that of the crystal phase to be nucleated.
A vitroceramic exhibits different physical and chemical properties than the glass material from which it originates. Vitroceramics also are isotropic, flexible as to shape, and their production cost is relatively low.
Some vitroceramics are fluorescent. Fluorescent materials convert incident light having a wavelength in one area of the spectrum into light having a wavelength in a different area of the spectrum. For example, when exposed to ultraviolet light, some fluorescent materials can convert that ultraviolet light into visible light. Some fluorescent materials can convert infrared light into visible light in a phenomenon known as up-conversion. In 1975, F. Auzel doped vitroceramics with rare earth metals. These vitroceramics converted infrared radiation into visible light (see F. Auzel, et al., Journal of Electrochemical Society 122(1)(1975), 101).
Some vitroceramics are photosensitive. When photosensitive vitroceramics are irradiated with short wavelength radiation such as ultraviolet radiation or X-rays, the optical properties of the material in the irradiated areas are modified. Photosensitive vitroceramics generally contain photosensitive metals such as copper (Cu), silver (Ag) and gold (Au). The photosensitive metals, upon exposure to the incident radiation, absorb that radiation. Upon heat treatment, the photosensitive metal particles are precipitated in the irradiated areas and serve as nucleation seeds for subsequent crystal formation. The resultant crystals change the color of the vitroceramic in those irradiated areas.
Photosensitive vitroceramics have been obtained as described in U.S. Pat. No. 2,651,145. This process for producing a photosensitive vitroceramic requires that a sodium-silica base glass containing silver as a photosensitive element be exposed to ultraviolet light. The silver absorbs the incident radiation. Next, a heating process is employed to generate a photographic image by precipitating silver particles in the irradiated areas. These silver particles, in turn, provide nucleation sites for the growth of NaF crystals. The NaF crystals are large enough to scatter visible light, resulting a white opaque image, which is opal-like in appearance.
While fluorescent vitroceramics are known in the art, and while photosensitive vitroceramics also are known in the art, it was not previously known to combine florescent properties with photosensitive properties in the same vitroceramic. Accordingly, it would be desirable to provide a vitroceramic having both fluorescent and photosensitive properties.
It would also be desirable to be able to control the degree of fluorescence of the vitroceramic.
It would further be desirable to be able to control the degree of fluorescence of the vitroceramic in selected areas of the vitroceramic.
It would further be desirable to provide a vitroceramic having both fluorescent and photosensitive properties for use in photography and fluorescent displays.
It would further be desirable to provide a vitroceramic in which the degree of fluorescence can be selectively controlled for use in computer memories.